Field of the Invention
The invention lies in the automotive field. More specifically, the present invention relates to a method and apparatus for transmitting digital data from a measurement station of an occupant protection system in a motor vehicle to an air bag control unit. The apparatus includes an air bag control unit, a measurement station connected to the air bag control unit, a direct current or direct voltage source in the air bag control unit for supplying the measurement station with a direct signal, first means in the measurement station for varying the direct signal in accordance with the data to be transmitted, and second means in the air bag control unit for detecting and evaluating changes in the direct signal as current changes. The measurement station is supplied with a direct signal from a direct current or direct voltage source of the air bag control unit and varies the direct signal in accordance with the data to be transmitted. The variations in the direct signal are then detected as current changes in the air bag control unit and evaluated.
For tripping circuits of occupant protection systems, which cause air bags or belt tensioners to be tripped in the event of an accident, for instance, signals must be transmitted from remotely located sensors (measurement stations) to a central control unit (air bag control unit). With increasing progress in the development of occupant protection systems, the associated sensors are being installed at more and more places in the vehicle. If the requisite short tripping times are to be adhered to, these remotely located sensors must be polled constantly, and the sensor signals must be transmitted reliably to the central control unit.
U.S. Pat. No. 5,712,784 to Fendt et al. (European patent application EP 0 693 401 A2) discloses a data transmission method in which pulse width modulated signals are generated by the affected remotely located sensors and are transmitted to the central control unit. The data information is encoded in the duty factor.
In a system for controlling an occupant protection device, such as an air bag, the entire system must be kept in operation for a minimum period after a collision in which the on-board electrical system and thus the energy supply to the system have been destroyed. To that end, in the air bag control unit, independent energy reserves, such as capacitors, are typically furnished for the individual circuit elements, so that this emergency operation can be assured for a minimum length of time. Such energy stores are not only expensive but also take up space.